An explosive composition

ABSTRACT

An explosive composition comprising a reagent that inhibits corrosion of a metal or metal alloy when the explosive composition comes into contact with the metal or metal alloy.

SUMMARY OF THE INVENTION

In general terms the present invention relates to inhibiting corrosioncaused by certain types of explosive composition with respect to metalsand metal alloys, in particular copper and copper alloys. The inventionhas applicability in the context of commercial operations whereexplosive compositions are used, such as mining and blasting operations.

BACKGROUND TO THE INVENTION

Commercial mining and blasting operations frequently use explosivecompositions that contain ammonium nitrate. The explosive composition isconveniently used in the form of a watergel or emulsion. These are wellknown and commonly used forms of explosive composition. The detonationsensitivity of the explosive can be increased by addition of sensitisingspecies, such as gas bubbles or solid agents such as microballoons andmicrospheres. Gas bubbles may be introduced into the explosivecomposition using a chemical gassing solution. A gasser solution is anaqueous solution comprising species that will react with one or morecomponents of the explosive composition (usually ammonium nitrate) togenerate gas bubbles. Various gasser solution technologies are known inthis regard. By way of example, the gasser solution may be an aqueoussolution of sodium nitrite. The gasser solution may include otheradditives that control the rate at which the gassing reaction proceedsas might be required depending on such things as prevailing conditions,e.g. temperature.

The sensitised explosive composition is commonly initiated using one ormore initiation devices. The initiation device will typically comprise adetonator, possibly used in conjunction with a booster charge (commonlyreferred to simply as a booster) in which the detonator is inserted.

Detonators typically take the form of an elongate cylinder (shell) thathouses a small explosive charge and componentry required to initiatethat charge. The cylinder is manufactured from a metal or a metal alloy,and aluminium, copper, brass (an alloy of copper and zinc) and steel arecommonly used. Copper and brass tend to be preferred because of ease ofmanufacture and because they can provide a detonator shell that hassuitably high physical strength to withstand shock and pressureencountered in extreme blasting conditions.

It has been observed however that blasthole conditions such as water,reactive ground, leachants, and physical contact with explosivecompositions may cause corrosion of copper and brass detonator shells.Various mechanisms may be responsible for this depending upon thecharacteristics of the explosive composition, the conditions under whichit is being used and the manner in which the explosive composition hasbeen sensitised to render it detonable.

If corrosion of a detonator shell is extensive, the physical strength ofthe shell can be impaired and this may impact on detonator efficacy.Corrosion may also compromise the integrity of the shell, therebyallowing ingress of external species such as water. In turn, this maycause the detonator to malfunction and misfire. This may havesignificant time and cost implications. Detonator misfire may also haveassociated safety issues since an undetonated explosive charge remainsin the blasthole.

There is on-going demand for higher mine productivity and the desire toundertake blasting operations in more challenging environments. Thismight involve such things as longer sleep times and/or blasting in hotand reactive ground. In such situations, the potential for corrosion ofdetonator shells may actually be increased.

Efforts to address corrosion in this context have included the use ofphysical barriers that are intended to isolate the outer surfaces of thedetonator shell from the environment in which it is used. Such effortshave included providing a lacquer, coating or polymeric sleeve on theexterior of the shell. However, these approaches had limited success oncopper and brass shells, the cost may be prohibitive and/or there may bemanufacturing difficulties.

It has also been suggested to form the detonator shell of a materialthat has suitable mechanical properties but that is more corrosionresistant than copper and brass. However, material selection is notstraightforward. For example, plastics and aluminium are too soft amaterial and will result in failure due to shock compression. Steel andother alloys may have suitable physical properties but shell manufacturemay be more difficult and the cost may be prohibitive. The use of copperand brass as the material for the detonator shell is therefore stillpreferred.

Against this background there remains the need to provide an alternativeand effective way of addressing corrosion of detonator shells in thiscontext.

SUMMARY OF THE INVENTION

The present invention seeks to address the corrosion problems discussedby providing modified explosive compositions that are less corrosivewith respect to metal/metal alloys. The present invention may be appliedto inhibit corrosion of copper and copper alloys and this is ofparticular interest. The invention will therefore be described withparticular reference to this. However, the principles of the inventionmay be more broadly applicable and may be applied to inhibit corrosionof other metals/metal alloys.

Accordingly, in one embodiment the invention provides an explosivecomposition comprising a reagent that inhibits corrosion of a metal ormetal alloy when the explosive composition comes into contact with themetal or metal alloy. Related to this the invention provides componentsused to produce such explosive compositions, to blasting systemsincluding the explosive compositions in combination with an initiatingdevice and to a method of blasting using the explosive compositions.These various embodiments will be better understood with reference tothe following more detailed discussion of the invention in the contextof inhibiting corrosion of copper and copper alloys.

Noting the emphasis explained above, embodiments of the presentinvention are based on identifying reagents that inhibit corrosion ofcopper and copper alloys (such as brass) when contacted with certaintypes of explosive composition. Accordingly, the invention provides anexplosive composition comprising a reagent that inhibits corrosion ofcopper and copper alloys when the explosive composition comes intocontact with copper or the copper alloy.

Embodiments of the invention relate to the production of such explosivecompositions and to components used in the production of such explosivecompositions. The invention also relates to blasting systems comprisingan explosive composition in accordance with the invention in combinationwith an initiating device having a copper or copper alloy surface thatin use will contact the explosive composition. The initiating devicewill generally be a detonator. In embodiments of the invention the shellof the detonator may be coated with a functional coating to providefurther enhanced corrosion resistance.

In another embodiment the present invention provides a method ofblasting in which an explosive composition in accordance with theinvention is provided in a blasthole and initiated using an initiationdevice. In the embodiments of particular interest this will be adetonator with a copper or copper alloy (usually brass) shell.

The invention may also be implemented using a combination of embodimentsas disclosed herein.

In describing the invention the expression “explosive composition”refers to an explosive that may be initiated using a conventionalinitiating system, for example using one or more detonators, possibly incombination with one or more boosters. The explosive composition willinvariably include sensitising species. In the present specificationsuch species will be introduced into what is referred to herein as an“explosive precursor”. The term “explosive precursor” is intended tomean a composition that contains stored chemical energy that can bereleased when the composition is suitably sensitized and detonated. Theexplosive precursor will usually be ammonium nitrate containing. It maybe a conventional emulsion explosive or a watergel explosiveformulation. The formulated explosive composition may also containammonium nitrate (AN) prill or ammonium nitrate/fuel oil (ANFO) prill.Such formulations are well known in the art.

Reagents useful in the present invention may be referred to simply as a“corrosion inhibitor” since that is the effect achieved in the contextof the invention. However, as will be explained, additives that areknown to inhibit corrosion of copper and copper alloys in other fieldsof use may not be useful in the context of the present invention sincethe chemical make-up of (sensitised) explosive compositions and thecorrosion causing species/reactions associated with such compositionsand their use can be complex. There are also various factors thatinfluence selection of a suitable reagent. The invention may beimplemented using one or more reagents that impart corrosion resistanceby the same or different mechanisms.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (orinformation derived from it), or to any matter which is known, is not,and should not be taken as an acknowledgment or admission or any form ofsuggestion that prior publication (or information derived from it) orknown matter forms part of the common general knowledge in the field ofendeavour to which this specification relates.

DETAILED DISCUSSION OF THE INVENTION

In accordance with the present invention enhanced corrosion resistanceas between an explosive composition and an initiating device including acopper or copper alloy that in use will come into contact with theexplosive composition may be achieved by modification of the explosivecomposition to include a functionally active reagent. This is believedto represent a significant departure from previous approaches where thefocus has been to provide a protective or passivating coating on thecopper or copper alloy itself. In accordance with the invention themanner in which the reagent is provided in the explosive composition mayvary depending upon various factors, including how the explosivecomposition has been sensitised, the type of composition and how it ismade, and how the explosive composition is delivered into a blastholeduring use.

As foreshadowed, the mechanism by which corrosion takes place may varydepending upon various factors. For example, conventional explosivecompositions may contain a number of species that are corrosive tocopper and copper alloys. These species may include, but are not limitedto, nitrate ions, nitrite ions, ammonia and various amines. Blastholeconditions and the presence of species such as chlorides, sulphates, orleachants may also encourage corrosion.

Ammonium and nitrate ions are present because the explosive compositionsto which the invention may be applied invariably contain ammoniumnitrate in aqueous form, and possibly as a solid additive in the formprills. The latter may be ammonium nitrate prill or ammonium nitrate infuel oil (ANFO) prill. When the explosive composition is sensitised withgas bubbles, nitrite ions may be present due to the chemistry of thegasser solution being used. Typically, the gasser solution containssodium nitrite. In emulsion explosive, emulsifiers are a source ofamines.

Ammonia is released from ammonium nitrate containing explosivecompositions under alkaline conditions. This may take place when gassersolution is added to the composition since the gasser solution may bebasic (a gasser solution based on sodium nitrite typically has a pH of8-9). Addition of the gasser solution may therefore result in anincrease in the overall pH of the system thereby causing release ofammonia. A gas-sensitised explosive composition may therefore exhibit arelatively fast rate of corrosion. Glass microspheres used forsensitisation can also react with ammonium nitrate in an explosivecomposition to yield ammonia thereby inducing corrosion.

The temperature at which the explosive composition is being used, thepresence of other species found in blasthole or mine water and leachantscausing pH changes can also influence the rate at which corrosion willtake place.

During the corrosion process copper forms copper complexes therebystripping copper from a surface. In a copper and/or copper alloystructure this may also lead to crack formation due to stress corrosioncracking. One possible reaction involving ammonium nitrate and ammoniawith copper is to form tetra-amino copper nitrate (TACN). This is a basecatalysed reaction that proceeds as follows:

Cu+NH₄NO₃+NH₃→(TACN)

In accordance with the invention it has been found that it is possibleto modify the otherwise corrosive nature of an explosive compositionwith respect to copper and copper alloys by inclusion in the compositionof a suitable reagent.

The reagent used in accordance with the present disclosure may inhibitcorrosion of a copper or copper alloy by a direct and/or indirectmechanism. Direct inhibition occurs when the reagent interacts with themetal/metal alloy itself to inhibit corrosion. For example, this mayoccur by the reagent protecting/passivating the metal/metal alloythereby reducing the availability of metal/metal alloy for reaction withcorrosive species. Indirect inhibition occurs when the reagentinfluences the properties of the corrosive environment to which themetal/metal alloy is exposed in order to render the environment lesscorrosive. Thus, the reagent may interact with corrosion causing speciesin an explosive composition to render them less corrosive. By way ofexample here reference may be made to gas sensitised explosivecompositions in which potentially corrosive gas bubbles are hydrophobicin nature. If the surface of the metal/metal alloy has hydrophobiccharacter, the gas bubbles will be attracted to the metal/metal alloythereby causing corrosion. In this case it may be desirable to use areagent that has the effect of changing the surface properties of thegas bubbles in order to reduce their surface hydrophobicity. This willreduce the affinity of the gas bubbles for the metal/metal alloy and indoing so inhibit corrosion. Additionally, or alternatively, the reagentmay modify the surface charge of the gas bubbles in order to reducetheir affinity for the surface of the metal/metal alloy. In anembodiment one or more reagents are used to provide corrosion inhibitionby a combination of direct and indirect mechanisms.

It has been found that benzotriazole (BTA) may be suitable for use asthe reagent in the various embodiments of the invention. In embodimentsBTA may inhibit corrosion by direct and/or indirect mechanisms.Structurally, BTA consists of benzene and triazole fused ring. BTA isbelieved to prevent undesirable surface reactions by forming aprotective layer on copper and brass. It is surprising that BTA acts asa corrosion inhibitor for copper in AN solutions.

The mechanism by which the complex forms is proposed and illustrated bythe following reactions:

Cu(s)+BTA(aq)→Cu:BTA(ads)

where Cu:BTA(ads) stands for BTA adsorbed onto a copper surface. In thepresence of oxidants or by anodic polarization it can be oxidised to aprotective complex:

Cu:BTA(ads)→Cu(I)BTA(s)+H⁺(aq)+e ⁻

From this reaction it can be seen that increase of BTA concentrationshifts the reaction towards formation of larger amount of the complexCu(I)BTA. An increase in pH has also been observed to favour formationof the complex.

The copper complex forms a film on the copper surface that preventscorrosive reactions at the surface due to species present in theexplosive composition. Adsorption of the inhibitor on the metal surfaceand film formation are believed to be important steps in the mechanismof corrosion inhibition.

It may also be possible to use derivatives of BTA as the reagent. Forexample it has been observed that the introduction of the substituentgroups (imidazole and its derivatives) has no effect on the mechanism ofthe inhibitive action while it does have an influence on inhibitionefficiency. Useful derivatives may include substituted BTA derivativesin which substituents are present on the benzene ring but not on thetriazole ring.

Examples of other compounds that may be useful as reagent in the contextof the present invention include imidazoles, triazoles,mercaptotriazoles, napthotriazoles, mercaptobenzimidazoles, azoles,triazines and tolyltriazines.

The present invention may be implemented using one or more suitablereagents and/or embodiments of the invention, therebyreducing/inhibiting corrosion of a number of different metals or metalalloys that may be in contact with an explosive composition. It may alsobe possible that a single reagent may be effective with respect to morethan one metal or metal alloy. It is possible that the efficacy of thereagent with respect to inhibition of corrosion may be increased by useof it in combination with other compounds.

BTA, useful BTA derivatives and other compounds that potentiate theeffect of these are commercially available. BTA may act as both a directand indirect inhibitor of corrosion. While acting directly on the metalor metal alloy as described above, BTA may also modify the surfaceproperties of gas bubbles thereby indirectly inhibiting corrosion.Without wishing to be bound by theory, this may involve a reduction inthe hydrophobicity of the gas bubbles and/or varying the charge of thegas bubbles. The intention is to reduce the affinity of the gas bubblesfor the surface to be protected against corrosion. Other reagents thatmay indirectly inhibit corrosion by the same mechanism include blockcopolymers, hydrophilic polymers and surfactants.

The amount of reagent to achieve effective results may vary dependingupon various factors including the mechanism by which corrosioninhibition is to be achieved, the propensity of the explosivecomposition to cause corrosion and the prevailing environmentalconditions in which the explosive composition is being used. The amountof reagent used may also be influenced by the solubility in the chosensolvent (carrier) and the molecular weight of the reagent. Broadlyspeaking the amount of reagent may be from 0.0001 to 1 wt % based on theweight of explosive composition.

In embodiments of the invention the reagent is required to be soluble inaqueous solution and the solubility of the reagent may also be arelevant consideration. The chosen reagent should not react with orotherwise interfere with the gassing reaction or the stability orintended function of the explosive composition being gassed. The reagentshould not interfere with formation or stability of the explosivecomposition and it should remain functionally active in the compositiononce formed. It is possible that the reagent may be included in anon-aqueous carrier depending upon its solubility and the manner inwhich the reagent is to be incorporated into an explosive composition.

In an embodiment, the reagent may be introduced into an explosiveprecursor or explosive composition in a component that is used toproduce the explosive precursor or explosive composition. It is alsopossible that the reagent may be introduced by use of a separatecomponent the sole function of which is to introduce the reagent. Theexplosive composition will comprise an explosive precursor andsensitising species. The reagent may be introduced into the explosiveprecursor before sensitising species are added to it, for example whenthe explosive precursor is being made. Alternatively, or additionally,the reagent may be introduced when sensitising species are beingincluded in the explosive precursor. Additionally, or alternatively, thereagent may be introduced into an explosive composition aftersensitisation of an explosive precursor has taken place. Thesepossibilities are discussed in more detail below.

In an embodiment the reagent may be provided in the explosive precursorwhen the latter is being produced. In this case, if the reagent is watersoluble, it may be incorporated into the explosive precursor in anaqueous component from which the explosive precursor is made. Forexample, in the case of an emulsion explosive the reagent may beincluded in the oxidiser salt solution from which the emulsion is made.To make the emulsion the salt solution and a fuel are mixed in thepresence of an emulsifier. A functionally effective amount of thereagent will be used. This approach may be useful for, but is notlimited to, explosive compositions that are not gas-sensitised. Suchexplosive compositions can be sensitized with solid density reducingagents, such as glass microspheres.

In an embodiment the reagent may be included in an explosive precursoror explosive composition during loading of the explosive precursor orexplosive composition into a blasthole. In the case of an explosiveprecursor sensitisation, for example by using a gasser solution, mayoccur during the loading process. When the explosive composition is anemulsion explosive individual, streams of explosive composition andreagent (typically provided in a suitable carrier) may be deliveredusing one or more loading hoses for mixing in the hose or as the streamsexit the end of the hose. A mixing device may be required if mixing isto take place as the streams exit the loading hose.

In an embodiment the reagent may be introduced into an explosiveprecursor or explosive composition via an aqueous solution that is usedto lubricate delivery of the explosive precursor or explosivecomposition through a blasthole loading hose. In this case the aqueoussolution will be provided as an annular stream around a stream ofexplosive precursor or explosive composition as it is being pumpedthrough a loading hose. The annular stream acts as a lubricant therebyimproving flow of the stream within the loading hose. The use of thistype of “water-ring” is known but not the inclusion in the aqueoussolution of a reagent to impart corrosion resistance. In this embodimentit is important that the aqueous solution used for lubrication is mixedwith and into the stream being pumped. This ensures suitabledistribution of the reagent. Mixing may be achieved using a mixing headprovided at the end of the loading hose from which the stream emerges.At that point the aqueous annular stream has served its role as alubricant.

In another embodiment the reagent may be included when an explosiveprecursor is being sensitised with gas bubbles. In this case the reagentmay be included in the gassing solution that is mixed with an explosiveprecursor in order to generate sensitising gas bubbles and yield anexplosive composition. The gasser solution may be mixed with explosiveprecursor before or during delivery into a blasthole. In the latter casethis may be achieved by providing the gasser solution as an annularstream around a stream of explosive precursor as it is being pumpedthrough a loading hose and into a blasthole. The annular stream acts asa lubricant and should be mixed with and into the explosive precursor toensure an even distribution of gas bubbles when the gassing reaction hastaken place.

Related to this there is provided a method of producing a gas-sensitisedexplosive composition with reduced propensity to cause corrosion ofcopper and copper alloys, the method comprising adding a gasser solutionto an explosive precursor in order to generate gas bubbles in theexplosive precursor, wherein the gasser solution comprises a reagentthat inhibits corrosion of copper and copper alloys when in contact withthe gas-sensitised explosive composition. Also provided is agas-sensitised explosive composition that has been produced by themethod.

An effective concentration of the reagent in the gasser solution may bedetermined taking into account the concentration of gasser solution thatis to be added to the explosive composition. The solubility of thereagent in the gasser solution may also determine the amount of reagentthat can be used. The gasser solution may itself be added to theexplosive composition in conventional amounts, for example from 0.25 to2.0 wt. % based on the total weight of the explosive precursor.

Further details are provided below with respect to one embodiment of thedisclosure related to the use of a reagent in the gasser solution:

-   -   1. A gasser solution for generating gas bubbles in an explosive        precursor to provide a gas-sensitised explosive composition, the        gasser solution comprising (a) one or more species that will        react with one or more species in the explosive precursor to        generate gas and (b) a reagent for a metal or metal alloy,        preferably for a copper or copper alloy. The corrosion inhibitor        should be soluble in the gasser solution.    -   2. The use of such a gasser solution for generating gas bubbles        in an explosive precursor and providing a gas-sensitised        explosive composition that exhibits reduced propensity to cause        corrosion of a metal or metal alloy preferably of a copper or        copper alloy.    -   3. A method of blasting in which this type of gas-sensitised        explosive composition is provided in a blasthole and initiated        using an initiation device.

Embodiments of the invention also involve using detonator shells thathave been pre-treated to provide a functional coating to assist withcorrosion resistance.

In an embodiment, when the reagent acts indirectly by reducing thesurface hydrophobicity of gas bubbles, the invention also contemplatespre-treating of a detonator shell to provide functionally activelayer(s) in order to achieve corrosion resistance.

With respect to pre-treating the detonator shell, it has been observedthat the affinity that the corrosive species have for water may have asignificant impact on the corrosion resistance that can be achieved.Thus, it has been found that when conventional gassing solutionchemistry is used in an explosive composition, coating the detonatorshell with a coating that should provide corrosion resistance but thatis hydrophobic in nature can actually increase the rate of corrosion.This is believed to be because the conventional gassing process producesgas bubbles that are themselves hydrophobic in nature and that aretherefore attracted to the hydrophobic surface. A high concentration ofgas bubbles at the shell surface may accelerate the rate of corrosionnotwithstanding the presence of the corrosion inhibitor. However, inthis case it may be desirable to provide in the explosive composition(be that via the gassing solution or otherwise) a reagent that has theeffect of changing the surface properties of the gas bubbles in order toreduce surface hydrophobicity. In turn this will lower the affinity ofthe gas bubbles for the hydrophobic coating of corrosion inhibitorprovided on surface of the detonator shell.

Thus, in an embodiment of the invention beneficial results may beachieved by the combined effect of providing a hydrophobic coating onthe detonator shell and by incorporating a suitable reagent in theexplosive composition to reduce the hydrophobic character of the gasbubbles produced during the gassing reaction. In an embodiment thereagent will be present as a component of the gasser solution that isused. An example of a suitable corrosion inhibitor for coating thedetonator shell and reagent for inclusion in the bulk of the explosivecomposition is BTA. In this case the reagent may also passivate anyareas of the detonator shell that have not been covered with corrosioninhibitor.

In another embodiment it has been observed that coating the shell of adetonator with a hydrophilic coating can reduce the rate of corrosionwhen conventional gassing solutions are used. This is believed to bebecause the gas bubbles produced are hydrophobic in nature and as suchwill be repelled from the surface of the detonator shell. However, ithas also been observed that hydrophilic coatings provided on thedetonator shell tend to swell over time in the presence of water. Thisis likely to occur with long sleep times of detonators loaded inblastholes prior to firing. Swelling of the coating can lead to thepresence of pores in the coating. Even though the walls of the coatingsurrounding the pores may be hydrophilic and water molecules mayactually plug the pores, pitting corrosion may still be problematic.This embodiment may therefore only be useful in dry environments or inwet environments where short sleep times are employed to minimiseswelling of the hydrophilic coating.

In accordance with another embodiment of the invention in an attempt tomitigate this issue, it may be desirable to provide a multi-layercoating on the detonator shell. Specifically, it may be desirable toprovide a first layer of a corrosion inhibitor on the detonator shell,for example a coating comprising BTA. This coating is preferablychemisorbed by the material of the detonator shell. A hydrophiliccoating is then applied over the top of that coating. In use thehydrophilic coating will repel hydrophobic gas bubbles. However, if thehydrophilic coating swells and pores develop, the corrosion inhibitorshould then prevent corrosive reactions at the surface of the shell. Asexplained above complex formation is believed to be responsible forpreventing corrosive reactions at the shell surface due to speciespresent in the explosive composition.

In this embodiment the layer of corrosion inhibitor provided on thedetonator shell may have hydrophobic character and thus attract gasbubbles. It may be desirable therefore to include in the explosivecomposition a reagent that will reduce the hydrophobicity of the gasbubbles that will be present, as described above.

In an embodiment a lacquer or varnish may be used to provide a coatingon the detonator shell to impart corrosion resistance. Preferably, thesurface of the shell should be clean before application of thelacquer/varnish. The lacquer/varnish may be applied to the shell bydipping the shell in the lacquer/varnish or by spraying. Varioussuitable lacquers/varnishes are commercially available and generallyinclude a polymeric resin dissolved in a suitable solvent and dosed witha suitable reagent to impart corrosion resistance. For example, productsare available comprising an acrylic ester resin dissolved in toluenewith BTA added to impart corrosion resistance.

In an embodiment a lacquer or varnish may be used to provide ahydrophilic coating to the shell of the detonator. Preferably, thesurface of the shell should be clean before application of thelacquer/varnish. The lacquer/varnish may be applied to the shell bydipping the shell in the lacquer/varnish or by spraying. Varioussuitable lacquers/varnishes are commercially available and generallyinclude a hydrophilic polymer provided in a suitable solvent or carrier.Epoxy and acrylic systems may be useful.

The suitability of a particular lacquer/varnish, the preferred method ofapplication and the optimum thickness may be varied to optimise results.

An additional embodiment of the invention is a blasting systemcomprising a detonator having a shell formed of copper or a copper alloyand an explosive composition in accordance with the invention, i.e. anexplosive composition modified to include a reagent that is functionallyeffective in reducing corrosion as described. In this embodiment outersurfaces of the shell of the detonator may comprises a coating thatinhibits corrosion of copper or copper alloy when in contact with theexplosive composition. When the explosive composition is sensitised withgas bubbles produced by mixing a gasser solution with an explosiveprecursor, the gasser solution may comprise a reagent that inhibitscorrosion of the detonator shell by reducing the affinity of the gasbubbles for the coating.

In a further embodiment outer surfaces of the shell of the detonator maycomprise a multi-layer coating. In this embodiment outer surfaces of theshell of the detonator comprise a multi-layer coating comprising a firsthydrophobic layer that is provided on outer surfaces of the detonatorshell and that passivates the outer surface with respect to corrosivespecies present in the explosive composition and a second hydrophiliclayer provided over the hydrophobic layer. In this embodiment, noaddition of a reagent to the explosive composition is necessary toprovide inhibition of corrosion.

Usually, in a blasting operation, one or more detonators are positionedin a blasthole (possibly in conjunction with a booster charge) withexplosive composition then being delivered into the blasthole and aroundthe detonator(s). Strictly speaking, to inhibit corrosion of a (copperor brass) detonator shell it is necessary for the corrosion inhibitor tobe present in that portion of the explosive composition that is indirect contact with the detonator shell. The invention could beimplemented to achieve that by varying the composition of the gassersolution (to include or omit corrosion inhibitor) that is injected intoan explosive composition as the explosive composition is being deliveredinto the blasthole. However, this adds a degree of complexity to theloading operation. Instead, it may be more practical to simply usegasser solution including corrosion inhibitor for the entirety ofexplosive composition being delivered into a blasthole.

Additional steps may be taken in an attempt to minimise corrosion of thedetonator shell. Thus, the detonator shell may be pre-treated with asuitable reagent to provide a protective layer on the shell. The reagentwill usually be provided in a suitable carrier. When the detonator ispositioned in a detonator well in a booster charge, the solution maysurround the detonator in the gap that exists between the outer surfaceof the shell and the internal surface of the detonator well.

The invention also provides a method of blasting in which an explosivecomposition in accordance with the invention is provided in a blastholeand initiated using a detonator comprising a shell formed of copper or acopper alloy. The method may utilise blasting systems in accordance withthe invention.

In relation to this embodiment it will be noted that in practice theremay be some considerable time (sometimes weeks) between loadingblastholes (with initiation devices and sensitised explosivecomposition) and firing of a blast. This may be the case for examplewhen the area being blasted and thus the number of blast holes is large.The present invention may allow detonators to be left in the potentiallycorrosive environment of a loaded blasthole for extended periods of timewithout corrosion that would otherwise effect detonator functionality.

The present invention may also find use in extreme ground/blastingconditions that are particularly aggressive with respect to detonatorcorrosion. The present invention may therefore allow blasting to beimplemented in situations that have otherwise proved difficult orimpossible.

Embodiments of the invention are now illustrated with reference to thefollowing non-limiting examples.

A standardised accelerated corrosion test was developed to assess therate of detonator shell corrosion, allowing a comparison of varioustreatments and reagents.

This accelerated test used was designed to corrode a brass detonatorshell within 18-24 hours, using a test solution containing 66 g ANS (50%ammonium nitrate solution) or emulsion, 33 g sodium nitrite solution(30% sodium nitrite) to mimic a commercial gasser formulation, andadditional corrosive salts 1 g sodium chloride and 0.150 g sodiumsulphate to mimic mine water leachants normally seen in a blasthole. Thetest conditions are far more aggressive than is normally seen in atypical blasthole.

Dummy detonators (operational printed circuit board, no explosivesecondary base charge and non-functional fusehead) were exposed to theaggressive test solution both at room temperature and at 40° C. forseveral weeks or until the integrity of the detonator was seen to fail.Detonators were continuously monitored both electronically (detonatorcircuitry functionality) and by visual and microscopic examination ofthe detonator shell. For the electronic functionality testing of thedetonators a logger is used.

Example 1 BTA in Gasser

Detonators were tested using the extremely aggressive test solutiondescribed above, both in the presence and absence of BTA (benzotriazole)in the gasser solution.

The non-BTA gasser was prepared by dissolving 30% w/w sodium nitrite inwater

For the BTA containing gasser 0.25% w/w of the BTA was dissolved inwater (Composition for a 100 g BTA gassing precursor solution: 0.25 gBTA; 99.75 g water). Once the BTA is completely dissolved (slow processdue to limited solubility of the compound) sodium nitrite is added toachieve a 30% w/w solution. (Composition for a 100 g gassing solution:30 g sodium nitrite; 70 g of BTA/water gassing precursor solution above)

The additional corrosive reagents, i.e. chloride and sulphate salts,were added to the gassing solution prior to addition of emulsion. Themixture was then stirred to prepare the suspension.

Three brass shell dummy detonators were placed into the suspension ofgasser/emulsion starting the corrosion reaction. The dummy detonatorswere monitored for performance as described above. In the absence of BTAsevere stress corrosion cracking was evident within one day leading tofailure of electronic functionality and structural failure of the dummydetonators, whereas the three dummy detonators placed into the BTAcontaining test solution showed structural integrity and full electronicfunctionality after 30 days.

Example 2 Coated/Lacquered Detonators

To protect the brass shell dummy detonators against general, pitting andstress corrosion cracking the detonators were coated with hydrophilicand hydrophobic lacquers and combinations thereof. The hydrophobiclacquer contains BTA as a corrosion inhibitor. Coatings were appliedusing a dipping process allowing the lacquer to cure for a period of 24hours before applying a second coating. The following lacquercombinations were exposed to the accelerated ANS/gasser test solution,described previously. The lacquered detonator corrosion experiments werecarried out at room temperature.

-   -   a) Dual hydrophilic lacquer combination    -   b) Hydrophilic/hydrophobic (BTA containing) lacquer combination    -   c) Hydrophobic (BTA containing)/hydrophilic lacquer combination

Detonators were continuously monitored both electronically (detonatorcircuitry functionality) and by visual and microscopic examination ofthe detonator shell.

Results obtained were as follows:

-   -   a) Extremely significant pitting and general corrosion,        electronic functionality compromised after 4 days.    -   b) Significant pitting and general corrosion, electronic        functionality compromised after 7 days.    -   c) No Stress cracking or pitting corrosion and full electronic        functionality observed after 30 days. (Detonators fully        operational after 40 days. Experiment was stopped.)

Comparative Example

In a separate experiment dummy detonators were washed with a BTA/watersolution (Composition for a 100 g solution: 0.25 g BTA; 99.75 g water)and allowed to completely air dry. Three detonators were then subjectedto the accelerated corrosion test described earlier (no BTA in gasser).The hydrophobic modified shell appeared to increase corrosion. This isbelieved to be due to drawing of hydrophobic, corrosive speciescontaining gas bubbles to the metal alloy interface, accelerating therate of corrosion. Within one day extreme pitting corrosion was observedand the dummy detonators failed electronically. These results shows thatBTA physically bonding to a brass metal surface, specifically the coppercomponent, is not sufficient in providing corrosion protection. Howeverwhen BTA is present in the gasser solution protection against corrosioncan be achieved. Without wishing to be bound by theory, it is believedthat the BTA in the gassing solution is believed to modify the corrosivegas bubbles by changing surface hydrophobicity and/or charge.

1. An explosive composition comprising a reagent that inhibits corrosion of a metal or metal alloy when the explosive composition comes into contact with the metal or metal alloy.
 2. An explosive composition according to claim 1, wherein the reagent inhibits corrosion of copper and copper alloys when the explosive composition comes into contact with copper or the copper alloy.
 3. An explosive composition according to claim 1, wherein the explosive composition comprises an explosive precursor and sensitising species.
 4. An explosive composition according to claim 3, wherein the explosive precursor comprises the reagent.
 5. An explosive composition according to claim 4, wherein the emulsion precursor is an emulsion produced by mixing an aqueous oxidizer salt solution with a fuel and emulsifier, and wherein the reagent is water soluble and provided in the aqueous oxidizer salt solution.
 6. An explosive composition according to claim 3, wherein the reagent is introduced into the explosive composition or explosive precursor via an aqueous lubricant that is used to lubricate delivery of the explosive composition or explosive precursor through a loading hose.
 7. An explosive composition according to claim 3, wherein the reagent is included in a gassing solution that is mixed with the explosive precursor in order to generate gas bubbles as the sensitising species.
 8. An explosive composition according to claim 1, wherein the reagent inhibits corrosion by a direct mechanism.
 9. An explosive composition according to claim 1, wherein the reagent inhibits corrosion by an indirect mechanism.
 10. An explosive composition according to claim 1, wherein one or more reagents are used to provide corrosion inhibition by a combination of direct and indirect mechanisms.
 11. A blasting system comprising a detonator having a shell formed of a metal or a metal alloy and an explosive composition as claimed in claim
 1. 12. A blasting system according to claim 11, wherein the detonator shell is formed of copper or a copper alloy.
 13. A blasting system according to claim 12, wherein outer surfaces of the shell of the detonator comprises a coating that inhibits corrosion of copper or copper alloy when in contact with the explosive composition.
 14. A blasting system according to claim 13, wherein the explosive composition is sensitised with gas bubbles produced by mixing a gasser solution with an explosive precursor, and wherein the gasser solution comprises a reagent that inhibits corrosion of the detonator shell by reducing the affinity of the gas bubbles for the coating.
 15. A blasting system according to claim 14, wherein outer surfaces of the shell of the detonator comprise a multi-layer coating comprising a first hydrophobic layer that is provided on outer surfaces of the detonator shell and that passivates the outer surface with respect to corrosive species present in the explosive composition and a second hydrophilic layer provided over the hydrophobic layer.
 16. A method of blasting in which an explosive composition as claimed in claim 1 is provided in a blasthole and initiated using a detonator.
 17. A method of producing an explosive composition as claimed in claim 1, wherein the reagent is introduced into an explosive precursor or explosive composition in a component that is used to produce the explosive precursor or explosive composition.
 18. A method of producing an explosive composition according to claim 1, wherein the reagent is included in a gassing solution that is mixed with an explosive precursor in order to generate sensitising gas bubbles and yield an explosive composition.
 19. A method of producing an explosive composition according to claim 1, which comprises introducing the reagent into the explosive composition or an explosive precursor via an aqueous solution that is used to lubricate delivery of the explosive composition or explosive precursor through a loading hose. 